The present invention relates to heat curable silicone rubber compositions. More particularly, the present invention relates to heat curable silicone rubber compositions containing an amount of certain co-curing agents effective to allow the use of less reinforcing filler while obtaining improved tear strength, suitable durometer and lower compression set. Heat curable silicone rubber compositions comprising (a) a polydiorganosiloxane base polymer, (b) an effective amount of reinforcing filler, and (c) an effective amount of free radical generating catalyst are well known in the art. It is appreciated by those skilled in the art that the amount of reinforcing filler employed is directly related to many of the physical properties of the resultant silicone rubber. For example, if the amount of reinforcing filler in a silicone rubber composition is increased, the durometer, tear strength and compression set will all generally increase somewhat. It is also recognized by those skilled in the art that if the filler loading is too high, the rubber becomes very difficult to process.
For most applications the artisan can obtain suitable properties simply by adjusting the filler level. However, in some applications, such as calendering solvent resistant silicone rubber to fabric, when sufficient filler is added to obtain the necessary durometer, the green strength or bulk modulus of the rubber is detrimental to calendering to fabric since it does not form the desired intimate, void-free compound/fabric interface. Moreover, the filler loading necessary to obtain a suitable durometer also increases the compression set beyond acceptable limits.
It has now been discovered that certain co-curing agents when added to heat curable silicone rubber compositions allow the use of less reinforcing filler while obtaining improved tear strength, substantially unchanged durometer, and lower compression set.
Konkle, U.S. Pat. No. 2,684,957, teaches that low compression set heat curable silicone rubber compositions can be made from (a) an organopolysiloxane, (b) a vulcanizing agent, and (c) from 0.75 to 8 parts by weight per 100 parts by weight of the organopolysiloxane of a cadmium compound selected from cadmium oxide, cadmium peroxide and cadmium carbonate.
Konkle and Talcott, U.S. Pat. No. 2,927,908, relates to the use of polytetrafluoroethylene to improve the tensile strength and tear resistance of fluorinated organopolysiloxane rubber.
Modic, U.S. Pat. No. 2,979,479, discloses that heat curable silicone rubber compositions having improved tear strength can be prepared from (a) an organopolysiloxane, (b) a finely divided silica filler, (c) a curing agent, and (d) a copolymer composed essentially of trimethylsiloxy groups and SiO.sub.2 groups, there being present from about 1.0 to 1.5 methyl groups per silicon atom.
Grabicki et al., U.S. Pat. No. 3,027,344, provides heat curable silicone rubber compositions characterized by improved tensile and tear strength comprising (a) an organopolysiloxane, (b) silica filler, (c) 1 to 20 parts of organic amine, and (d) 1 to 20 parts of a hydroxy organosilane selected from diphenylsilanediol and triphenylsilanol.
Wada et al., U.S. Pat. No. 3,652,475, teaches heat curable silicone rubber compositions comprising (a) 100 parts by weight of polydiorganosiloxane having a degree of polymerization of at least 3000 and containing at most 0.3 mole percent vinyl groups, (b) from 1 to 10 parts by weight of polydiorganosiloxane having a degree of polymerization of at least 3000 and containing from 5 to 20 mole percent vinyl groups, (c) from 0.5 to 5 parts by weight of polydiorganosiloxane having a degree of polymerization of from 10 to 1000 and containing from 5 to 90 mole percent vinyl units, (d) 20 to 200 parts by weight of silica filler, and (e) a catalytic amount of organic peroxide; said compositions having improved tear strength and superior compression set.
None of the foregoing references teach or suggest that improved tear strength, acceptable durometer and lower compression set can be obtained by using less reinforcing filler, but including a co-curing agent such as trimethylol propane trimethacrylate or triallyl trimellitate.